1. Field of the Invention
The present invention relates to an image forming apparatus which performs a preliminary heating operation for keeping the temperature of a fixing device equal to or greater than a predetermined temperature in a standby-mode where an image forming request is waited for.
2. Description of the Related Art
An increase in the speed and the number of colors used in an image forming apparatus such as a copier or a printer has been accomplished in recent years. In such a high-speed printer or color printer, a control target temperature of a fixing device should be set high at the time of fixing a toner image formed on a recording sheet by heating. In such a high-speed printer or color printer, the power consumption in parts other than the fixing device in the apparatus is great and the power which can be assigned to the fixing device decreases. When the power to be assigned to the fixing device decreases, the time period increases until the fixing device rises to a temperature at which an image on the recording sheet is fixed after an image forming request is input to the apparatus.
However, the time period until a first recording sheet is discharged after the image forming request is input, that is, a so-called first printout time (hereinafter, referred to as “FPOT”), should be as short as possible. Accordingly, as described in Japanese Patent Application Laid-Open No. 2006-98998, a method may be employed in which the power consumption for starting up the fixing device to a fixable state, in which an unfixed image on the recording medium can be fixed, is reduced without lengthening the FPOT by warming the fixing device to some extent in a standby-mode to wait for a request of an image forming operation. The time period until the temperature of the fixing device rises to the fixable temperature after the image forming request is input greatly depends on the temperatures of other elements of the fixing device, particularly, the temperature of a pressure roller, as well as the temperature of a heating unit. However, in the image forming apparatus according to the related art in which a control of warming the fixing device in the standby-mode is made, it is controlled to keep the temperature of the heating unit at the control target temperature in the standby-mode as soon as possible, when the temperature of the heating unit falls to the control target temperature in the standby-mode just after a printing operation is ended and the operation mode is switched to the standby-mode. In the apparatus for controlling warming of the fixing device in the standby-mode, much power is consumed and it is thus preferable that the power consumption be suppressed as much as possible even in the standby-mode. However, Applicants have found that the power consumption in the standby-mode can be further suppressed in the above-mentioned control.